向后传递数据时，在展开可选值时意外发现 nil

Posted 2023-04-19

【中文标题】向后传递数据时，在展开可选值时意外发现 nil

【英文标题】：Unexpectedly found nil while unwrapping an Optional value when passing data backward

【发布时间】：2019-04-18 08:36:06

【问题描述】：

我已经学习 Swift 和 Xcode 大约 3 周了，我正在做一个 Todo 应用程序只是为了练习。但是，当我尝试将新项目添加到我的待办事项列表时，出现了“在展开可选值时意外发现 nil”的错误。

我有由 Main.storyboard 创建的主屏幕和主屏幕上的添加按钮。当单击该按钮时，它会转到我通过 Eureka 框架创建的新 ViewController，用户可以在该表单中输入一些信息，如标题、描述、类别并将它们传递回主屏幕。我使用委托协议来传递数据，当我单击“保存项目”按钮时，应用程序崩溃了，它说

“致命错误：在展开可选值时意外发现 nil”。

代码如下：

import Foundation
import Eureka
protocol CanReceive 

    func dataReceived(data: ToDo)


class AddItemViewController : FormViewController 

    var delegate : CanReceive?

    var todoItem : ToDo?

    static let dateFormatter: DateFormatter = 
        let formatter = DateFormatter()
        formatter.dateFormat = "MMM d yyyy, h:mm a"
        return formatter
    ()

    override func viewDidLoad() 
        super.viewDidLoad()
        form +++ Section()
            <<< TextRow()
                $0.title = "Title"
                $0.placeholder = "Enter text here"
                $0.onChange  [unowned self] row in
                    self.todoItem?.title = row.value
                
            
            <<< TextRow()
                $0.title = "Description"
                $0.placeholder = "Give some description"
                $0.onChange  [unowned self] row in
                    self.todoItem?.description = row.value
                
            

            <<< AlertRow<String>() 
                $0.title = "Category"
                $0.selectorTitle = "Select the category"

                $0.options = ["Personal ????", "Home ????", "Work ????", "Play ????", "Health ????????‍♀️" , "Other"]
                $0.onChange  [unowned self] row in
                    self.todoItem?.category = row.value
                
                

            +++ Section() section in
                section.header = 
                    var header = HeaderFooterView<UIView>(.callback(
                        let button = UIButton(frame: CGRect(x: 100, y: 100, width: 50, height: 50))
                        button.backgroundColor = .darkGray
                        button.setTitle("Save Item", for: .normal)
                        button.addTarget(self, action: #selector(self.buttonAction), for: .touchUpInside)

                        return button
                    ))
                    header.height =  50 
                    return header
                ()
        
    

    @objc func buttonAction(sender: UIButton!) 
        delegate?.dataReceived(data: todoItem!)
        self.dismiss(animated: true, completion: nil)

        print("Button tapped")
    

Todo 类型定义为：

struct ToDo 

    var title: String?
    var description: String?
    var category : String?

这里是控制 main.storyboard 的 ViewController：

import UIKit

let storyBoard : UIStoryboard = UIStoryboard(name: "Main", bundle:nil)




class ViewController: UIViewController, UITableViewDelegate, UITableViewDataSource, CanReceive 

    var todoList = [ToDo]()
    let cellId = "CellId"
    let test = ["Row1", "Row2", "Row3"]
    let imageName = ["anchor", "arrow-down", "aperture"]
    @IBOutlet weak var ImageTop: UIImageView!
    @IBOutlet weak var tableView: UITableView!

    override func viewDidLoad() 
        super.viewDidLoad()
        self.tableView.register(UINib(nibName: "CustomTableViewCell", bundle: nil), forCellReuseIdentifier: cellId)
        configureTableView()
        ImageTop.image = UIImage(named: "todo-background")


    

    //data source method

    func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int 
        return todoList.count
    

    func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell 
        let cell = tableView.dequeueReusableCell(withIdentifier: cellId, for: indexPath) as! CustomTableViewCell
        cell.categoryLabel?.text = todoList[indexPath.row].category
        cell.descriptionLabel.text = todoList[indexPath.row].description
        var imageCategory = ""
        switch todoList[indexPath.row].category 
        case "Personal ????":
            imageCategory = "Personal"
        case "Home ????":
            imageCategory = "Home"
        case "Work ????":
            imageCategory = "Work"
        case "Play ????":
            imageCategory = "Play"
        case "Health ????????‍♀️":
            imageCategory = "Health"
        case "Other":
            imageCategory = "Other"
        default:
            break
        
        cell.imageCategory?.image = UIImage(named: imageCategory)
        return cell
    

    func configureTableView() 
        self.tableView.rowHeight = 80
        self.tableView.separatorStyle = .none

    

    func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) 
        print("Did select row at index \(indexPath.row)")
        print("Row height is: \(tableView.rowHeight)")
    
    @IBAction func addButtonPressed(_ sender: RoundButton) 
        let addItemViewController = AddItemViewController()
        addItemViewController.delegate = self
        self.present(addItemViewController, animated:true, completion:nil)
    

 //Delegate 
    func dataReceived(data: ToDo) 
        todoList.append(data)
        return
    

感谢您的宝贵时间，如果我的问题如此愚蠢，我们深表歉意。

【问题讨论】：

哪一行报错了？

进入表单并点击Save Item按钮后，应用程序崩溃并说Unexpectedly found nil while unwrapping an Optional value，我将添加图像以进行更多说明。

你应该在 Xcode 中设置一个异常断点，它会告诉你哪一行正在崩溃。顺便说一句，你为什么在闭包中使用unowned self 而不是weak self？

我删除了这些图片，因为它们无关紧要。

What does "fatal error: unexpectedly found nil while unwrapping an Optional value" mean?的可能重复

【参考方案1】：

发生错误是因为在AddItemViewController 中，属性todoItem 已声明但未初始化。

我建议对三个属性使用临时变量，并在按下按钮时创建一个ToDo 实例

class AddItemViewController : FormViewController 

    var delegate : CanReceive?

    var title, description, category : String?

    static let dateFormatter: DateFormatter = 
        let formatter = DateFormatter()
        formatter.dateFormat = "MMM d yyyy, h:mm a"
        return formatter
    ()

    override func viewDidLoad() 
        super.viewDidLoad()
        form +++ Section()
            <<< TextRow()
                $0.title = "Title"
                $0.placeholder = "Enter text here"
                $0.onChange  [unowned self] row in
                    self.title = row.value
                
            
            <<< TextRow()
                $0.title = "Description"
                $0.placeholder = "Give some description"
                $0.onChange  [unowned self] row in
                    self.description = row.value
                
            

            <<< AlertRow<String>() 
                $0.title = "Category"
                $0.selectorTitle = "Select the category"

                $0.options = ["Personal ?", "Home ?", "Work ?", "Play ?", "Health ??‍♀️" , "Other"]
                $0.onChange  [unowned self] row in
                    self.category = row.value
                
                

            +++ Section() section in
                section.header = 
                    var header = HeaderFooterView<UIView>(.callback(
                        let button = UIButton(frame: CGRect(x: 100, y: 100, width: 50, height: 50))
                        button.backgroundColor = .darkGray
                        button.setTitle("Save Item", for: .normal)
                        button.addTarget(self, action: #selector(self.buttonAction), for: .touchUpInside)

                        return button
                    ))
                    header.height =  50 
                    return header
                ()
        
    

    @objc func buttonAction(sender: UIButton)  // no implicit unwrapped optional
        let todoItem = ToDo(title: title, description: description, category : category)
        delegate?.dataReceived(data: todoItem)
        self.dismiss(animated: true, completion: nil)

        print("Button tapped")
    

【参考方案2】：

我认为这行delegate?.dataReceived(data: todoItem!) 导致了您的错误，因为您在未初始化时隐式地解开了 todoItem